The present invention relates to optical measurements of thickness and other physical properties of specimens of thin-films or other material forms typically provided on a substrate and, more particularly, to methods and apparatus to perform such measurements over entire specimens rapidly in obtaining the desired specimen characteristics in a single measurement step.
Many industrial manufacturing processes require measurements of thickness and other physical properties of specimens of thin-films supported on a substrate, typically formed of a solid material, or specimens of gaseous or liquid materials supported in an appropriate manner or some combination thereof. In general, for a film measurement based on light transmission therethrough, for instance, a light source, perhaps a broadband white light (“WL”) source, provides light transmitted through an illumination optics system to impinge collimated or focused on, and then pass through, the film specimen under test. The light emerging from the other side of the specimen is collected by a collection optics system and directed to a detector which in turn develops corresponding photovoltage response signals to be amplified and used in subsequent signal processing steps. Similarly, for a film measurement based on reflection of light from the film surface or surfaces, such light provided by an illumination optics system is directed collimated or focused onto, and then reflected from, the surface or surfaces of the specimen under test, and the reflected light is thereafter collected by a collection optics system for a detector that provides the signals for subsequent signal processing. In this process, the transmitted light may be directed through a beamsplitter and reflected back to the same beamsplitter from the surface of the specimen to conveniently provide and collect the light on paths at least initially perpendicular to that surface.
More particularly, many semiconductor fabrication processes, for instance, involve deposition and removal of thin-films of various materials. As an example, chemical-mechanical polishing (“CMP”) is a process widely used to planarize both metals, such as copper, and dielectrics, such as silicon dioxide. In this process, the thickness of the films being polished, especially the uniformity of thickness thereof over the surfaces of the entire specimens, is critical to the fabrication yield of the resulting products. In photonics manufacturing processes, such as those for the fabrication of semiconductor laser diodes (“LDs”), vertical-cavity surface emitting lasers (“VCSELs”), dense wavelength-division multiplexing (“DWDM”) film filters, optical waveguides, etc., the measurement and control of film thicknesses again are critical to the quality of the products resulting from fabrication.
There are two categories of technology methods for measuring the distribution of film thickness and other physical characteristics over the entire specimen: the full-wafer imaging method and the scanning method. One imaging spectrometer for measuring film thickness over an entire semiconductor wafer uses a broadband light source, such as a halogen lamp, in combination with a rotating filter wheel assembly, to form a monochromator. The associated optical system enables illumination of the wafer by a collimated beam. This allows collection of spectroscopic reflectance data from the entire wafer relatively rapidly. Nevertheless, there are several disadvantages limiting the performance of this system. First, the filter wheel assembly requires mechanical movements over a large range. This usually is undesirable for precise measurements such as those in semiconductor and photonics industries, for instance. This kind of mechanical movement is usually slow, and so the speed of the system, therefore, is limited. In addition, such movements are noisy, may cause substantial vibrations, and are a possible source for generating small particles due to material wear or other sources. Also, the bandwidth of the filters in the rotating filter wheel assembly is in the range of 3 to 5 nm, which is too wide for precise measurements. Another issue is the beam collimating system. This system uses either a lens or an off-axis parabolic reflecting mirror to provide a collimated illumination beam. As the diameter of specimens desired to be tested continues increasing in practical fabrication processes, large aperture lenses and off-axis parabolic mirrors are required. Both can be very expensive and require considerable complexity to correct the optical aberrations occurring in them. These considerations become acute, for instance, in fabricating semiconductor logic and memory devices for which 300 mm wafers are currently commonly used. In display devices, such as liquid-crystal (“LC”) display panels, the sizes encountered are even larger.
Several other designs have been disclosed related to this art including the use of a spherical vacuum chuck to deform a semiconductor wafer into serving as a concave mirror. In this way, the illumination light beam is reflected backward in the direction of the incident light beam. As a result, a relatively small lens may be used to illuminate a large wafer. In practical implementation, however, this arrangement can induce considerable difficulty. First, a wafer so deformed may not result in forming an optical-grade reflective mirror, especially for large diameter (aperture) wafers, and so the resulting defocusing and spherical aberrations may, as a practical matter, defeat this method for precise measurements. Moreover, in most semiconductor fabrication processes, bending the wafers in process is highly undesirable, and often, unacceptable. In another arrangement, a filter wheel assembly is placed in front of a charge-coupled device (“CCD”) camera. A ground glass screen is placed in front of the broadband light source to diffuse the illumination light. This system allows more reflected light to be collected. However, because the angle of incidence (“AOI”) of the diffused illumination light is, as a result, undefined (with certain random components), a portion of the reflected light related to the diffused illumination light, instead of contributing to the useful signal, contributes to unwanted noise. This result, for practical applications, is disadvantageous.
Another system measures thick wafers by using an imaging Fourier interferometer based on an infrared light source to form a Fourier transform infrared (“FTIR”) spectrometer. The semiconductor wafer specimens also need to be deformed by a spherical vacuum chuck.
Typically, a complete wafer imaging system with a collimating objective is able to provide a spatial resolution at the semiconductor wafer surface plane of about 200 microns (μm) per pixel. To achieve higher resolution (e.g., 5 to 10 μm per pixel, for instance), a system can use a scanning microscope objective lense or a sparse array of lenses. This arrangement, in fact, can be regarded as a technology belonging to the second technology category indicated above, scanning systems for measuring the distribution of film thickness and other physical properties over the surface of the entire specimen.
In the category of scanning systems, conventional film measurement technologies, such as reflectometry and ellipsometry, are used to perform single-point measurements. The distribution of the characteristics of interest of the film specimens is obtained either by placing the semiconductor wafer on a scanning stage to be moved past the optical head, or by moving the optical head to scan the entire fixed position semiconductor wafer, such as an ellipsometer with a beam deflector, which translates the optical head, allowing point-to-point and site-to-site measurements.
Another type of scanning system is a microscope objective-based spectroreflectometer. Commercially available instruments have been developed, based on the same scanning measurement principle.
A further slightly different system is based on a line-scan spectroreflectometric principle. Instead of the above point-to-point scanning-measuring scheme, this prior art uses a cylinder lense to form a line of illumination across the entire wafer. The wafer is then translated in the direction perpendicular to this illumination line. In this line-scan method, the spectral reflectance data over the entire wafer are obtained. Detection is provided by a two-dimensional CCD array that is used to collect the spectral reflectance data. One dimension of the CCD, in combination with a diffraction grating, is used to measure dispersed light. Light at different wavelengths corresponding to different angles is collected by different pixels of the CCD array in this dimension. This arrangement, actually, is the fundamental constraint to prevent this system from complete wafer imaging, because only one dimension of the CCD can be used to distinguish spatial positions on the wafer surface plane. This, in turn, makes the line-scan necessary.
In these inventions, a key device is a spectrometer which measures the distribution of light power reflected from or transmitted through the film specimen under test over a given spectral range. The spectrometer, constructed with light-emitting diodes (“LEDs”), has been developed in a variety of configurations. Two LEDs at different wavelengths are used to illuminate a film sample under test. The light reflected from the sample is collected and provided to a photodetector, and the corresponding reflectance spectrum data are analyzed. In this system, the LEDs are considered to be monochromatic light sources and no further wavelength dispersion means are used. LEDs, with a fill-width at half-maximum value (“FWHM”) spectral width in the range of ΔλFWHM=20 to 100 nm, may be considered narrowband or monochromatic for certain applications. Most precise measurement applications, such as those in semiconductor and photonics metrology, require much higher wavelength resolution, i.e. narrower spectral width spreads. A straightforward method to overcome this issue is to combine LEDs with a monochromator. In this way, the LED assembly is nothing more than a broadband light source which replaces conventional sources such as halogen lamps, for instance. Typically, a monochromator consists of a dispersive element, such as a diffraction grating, and a scanning output slit to select a specific wavelength. Thus, the dispersive element can be a holographic, concave, reflective grating with Au or Al coatings, the light source can be a set of 80 LEDs in a two-dimensional array of 4 rows and 20 columns, and the output slit is driven by a stepper motor, to scan over a spectrum range from blue (470 nm) to mid-infrared (40,000 nm).
Wavelength scanning also may by realized by an oscillating grating. Light from an LED array, in the spectrum range of 1100 to 2600 nm, with Δλ=100 nm, is coupled to the entrance slit of a monochromator via optical fibers. Both the entrance and exit slits are fixed. The wavelength selection is realized by a motor-driven concave reflective diffraction grating.
In this class of prior art, the LEDs only serve as a robust, long lifetime, and cheap broadband light sources. The fundamental operation principle is the same as that in the conventional spectrometer with a scanning monochromator. The mechanical movements, which are necessary for the operation of that type of spectrometer, however, are highly undesirable in many applications, for instance in semiconductor and photonics metrology. Mechanical movement usually are slow, noisy, bulky and expensive. In addition, they may be potentially sources from which particles are generated, which are extremely detrimental to wafer chip yield in semiconductor manufacturing processes.
One alternative to the LED-monochromator type spectrometer provides wavelength selection through an arrangement of spatial positions of each of the LEDs in the LED array. Both the concave reflective grating used and the exit slit are fixed. There is no entrance slit, and the active area of each LED is used as the equivalent of an entrance slit. As each LED is turned on, the angular positions of the LED and the exit slit in reference to the normal of the diffraction grating reflective surface changes. This, in turn, is equivalent to a virtually “scanning” entrance slit. As a result, the fixed exit slit will allow different wavelengths to be selected. Because of the finite size of the LED active area, the wavelength resolution of this spectrometer is not high, i.e., ΔλFWHM is in the range of 11.5 to 13.5 nm. This is suitable for applications in chemical technology and biotechnology, such as for foodstuff inspection, for instance. For precise measurements, such as those in semiconductor and photonics metrology, this wavelength resolution is not sufficient.
There are other applications utilizing tunable filters, e.g., acousto-optic tunable filters (“AOTFs”), for wavelength selection. They usually operate in the near infrared (“NIR”) to result, for example, in a NIR spectrometer in the wavelength range above 900 nm to 3900 mn. Uses are usually in chemical technology and biotechnology related industries, such as food and dairy, pharmaceutical and agriculture, for instance. An example is a spectrometer on a chip, made of lithium niobate (or LiNbO3) formed on a silicon substrate, integrating an LED array and an AOTF, for use in analyzing gas and fluid samples.
Usually, LED-based spectrometers are used for chemical technology and biotechnology related applications, such as concentration analysis, for instance. In those applications, the requirements for the wavelength resolution and wavelength stability can be relaxed. In precise measurements, such as those in semiconductor and photonics metrology, requirements for those parameters are much more strict. In semiconductor film thickness measurements, for instance, most commercially available tools are capable of measuring multilayer film stacks thicknesses with sub-Angstrom 3-σ precision for thin-films, and better than 1% precision for thick films. Existing LED spectrometers, with spectral widths in the range of a couple of nanometers to more than ten nanometers, are unable to meet the wavelength resolution requirements for precise measurement applications.
Furthermore, both the wavelength and the intensity of light emitted by LEDs depend strongly on temperature variations due to the dependence of the emission energy spectrum thereof on the LED temperature. Theoretically, this emission energy spectrum is defined by the Boltzmann distribution which is proportional to exp(−kT), where k is the Boltzmann constant and T is the temperature, on the high photon energy side of the spectrum, and the density of states, which is proportional to (E−Eg)1/2, where E is the energy and Eg is the bandgap energy, on the low photon energy side. As a result, increasing temperature will shift the emitted light wavelengths to the longer side (red-shift), and concurrently decreases the intensity. Without proper calibration and stabilization means, LED spectrometers are unable to meet the requirements for precise measurements mentioned above.